There are several inherent problems associated to the use of carboxylic acid comonomers including the following: a) the acidity of the acid comonomer can cause corrosion problems, b) the high reactivity and self-polymerizability of the acid comonomers require them to be stored at low temperature conditions and preferably in the liquid state, c) carboxylic acids require stabilization with either nitroxyl containing inhibitors, like phenolthiazine, or mono-methyl-ether of hydroquinone (MEHQ) activated with oxygen, and the nitroxyl containing inhibitors can inhibit or retard polymerization, while the oxygen present in the MEHQ can act as an initiator when applied at elevated temperatures.
Further complications could arise from the use of carboxylic acid monomers in high pressure free radical polymerization, which is carried at pressures preferably above 1000 bar, more preferably above 1500 bar and most preferably above 2000 bar. At these elevated pressures, the melting point of carboxylic acid monomer is significantly increased. In the high pressure process, the carboxylic acid has to be fed to the process in the liquid state, and is moved through the compression, the polymerization, and the separation sections, preferably in the gaseous phase (dissolved and diluted in gaseous or super critical ethylene), in order to avoid self polymerization of liquid or solid carboxylic acid monomer to carboxylic acid homopolymer.
The high pressure polymerization process makes use of reciprocating plunger compressors to pressurize the ethylene/carboxylic acid monomer feed and recycle streams. It is known that application of carboxylic acid monomer negatively affects the reliability of rotating and reciprocating equipment, due to its corrosion and self polymerization potential. The formation of carboxylic acid homopolymer in the sealing ring or bearing areas of the rotating and moving equipment items in compressors, plunger pumps, agitators etc, could hinder, or stop, lubrication, and could cause friction and extra heat formation, which could lead to equipment failure. Compressor reliability could be improved by decreasing the carboxylic acid monomer level in the ethylene stream(s) to be compressed through feeding all, or a part of, the make-up carboxylic acid monomer directly at high pressure to the reactor.
Feeding carboxylic acid monomer to a high pressure reactor requires a high pressure diaphragm or reciprocating plunger pump. These pumps have to be operated at higher temperatures in order to avoid solidification of the carboxylic acid monomer. High pressure pump operation with carboxylic acid monomer is challenging, unreliable, if not impossible, by the higher temperature operation required to avoid solidification of the carboxylic acid monomer, the compression energy (further heat up) and the self-polymerization potential. Thus there remains a need to pump carboxylic acid monomer reliably at lower temperatures to pressures above 1000 bar.
Conventional polymerization processes, include those described in U.S. Pat. Nos. 5,028,674, 4,417,035, 4,351,931, 5,057,593, 4,599,392, 4,252,924, 365,874 and GB1176696A. However, none of these processes meet the requirements and needs, discussed above, for high pressure free radical polymerizations. All these requirements and unmet needs have been met by the following invention. The invention provides for the improved feeding of carboxylic acid monomer with the help of a polar compound, which reduces significantly the melting point of carboxylic acid mixtures, thus allowing direct feeding of the carboxylic acid monomer to the reactor and improving secondary compressor reliability. Furthermore the invention allows wide melt index control capability, while the reactor and compressor phase equilibria can be positively influenced for a wide range (MI and carboxylic acid monomer level) of ethylene-carboxylic acid copolymers.